In recent years, IP-related protocol set including IP (Internet Protocol), SIP (Session Initiation Protocol) which performs call control between radio communication terminals, and the like has been used in a radio communication network such as a mobile communications network (see, RFC3261 “SIP: Session Initiation Protocol”).
In addition, in a mobile communication network using the protocol set, “group call” has been realized in which a radio communication terminal (for example, a cellular phone terminal) calls multiple different radio communication terminals to form a group of the radio communication terminals. In the group call, radio communication terminals participating in a group can perform voice communications, and transmit and receive messages (for example, chat) with other radio communication terminals participating in the group.